The present invention relates to an apparatus for producing high-temperature ultrapure water having a minimal amount of dissolved oxygen and no increment of impurities such as total organic carbon (TOC), metallic ions, or the like, which is provided for improvement in a final cleansing procedure of components during semiconductor manufacturing process and also, to parts and materials of pipings of such apparatus.
So-called ultrapure water is used in the manufacture of semiconductors. It is desired that such ultrapure water contain the least possible amount of impurities including various ions, total organic carbon, fine particles, or the like. Because the ultrapure water having the least impurities is highly active as a solvent, components materials of the system for handling it are primarily made of resin having low leachability such as PVC (polyvinl chloride), PFA (Teflon.TM.), PVDF (polyvinylidene fluoride), PEEK (polyetherether-ketone), and so on. Also, surface polished stainless steel is utilized when the ultrapure water has to be heated up for the purpose of sterilization or the like. However, during the heating, some impurities are leached from those materials and increase up to about ten times the amount present at an ambient temperature.
As very detailed work is required for processing integration of semiconductors, the spontaneous development of an oxide film (native silicon oxide) on the surface of a semiconductor is significant because it increases the contact resistance on the surface of a wafer and thus results in a decline in the performance of integrated circuits. Such an oxide film is uniformly developed on the surface of each wafer by the dissolved oxygen in ultrapure water and causes defects in the arrangement of high-density circuit connection on the wafer. The dissolved oxygen in the ultrapure water can be reduced to only 30 to 50 ppb by a vacuum deaeration procedure. Hence, an improved method of removing a greater amount of dissolved oxygen from ultrapure water is much desired in this technical field.
At the final cleansing stage of a conventional process of producing semiconductors, a heated inert gas such as argon gas or nitrogen gas is used for removal and drying out of the ultrapure water. To improve the cleansing procedure, it is preferable to maintain both the cleansing ultrapure water and the wafer at a temperature of 80.degree. to 90.degree. C.
In the operation of a prior art ultrapure water producing system (see FIG. 3), the raw water is first fed into a pretreatment process 34 of a primary ultrapure water system 33 for removing suspended solids by flocculation, settling and filtering, and passed to a primary water purifying process 35 for removing dissolved salts and organic materials by reverse osmosis and ion exchange. The resultant primary ultrapure water from which a majority of the impurities have been removed is then transferred to an ultrapure water reservoir 32 in a secondary ultrapure water system 31.
The primary ultrapure water is then delivered by an ultrapure water pump 36 from the ultrapure water reservoir 32 through a total organic carbon destruction device 37, a polisher 38, and an ultrafilter 39 for removing further impurities. A portion of the secondary ultrapure water at an ambient temperature is supplied to a use point (point of use) 40 while the remaining portion is returned back to the ultrapure water reservoir 32 through a valve 30. Also, another portion of the secondary ultrapure water which is passed from the ultrafilter 39 flows into a branch line 41 and is heated by a heater 43 mounted in an ultrapure water heating device 42 up to a predetermined temperature and then fed into a high-temperature use point 44 for cleansing wafers.
The ultrapure water heating device 42 employs a material such as tetrafluoride resin, synthetic quartz, or the like as the parts which are in direct contact with the ultrapure water. The pipings for passing the high-temperature ultrapure water are made of difluoride resin and the like.
However, when the high-temperature ultrapure water is in direct contact with the materials of the devices and pipings, then organic matters, fluorine, silica, and so forth are leached out thereby increasing the amount of impurities in the ultrapure water. Also, the ultrapure water heating device 42 is capable of heating the secondary ultrapure water but not removing dissolved oxygen; therefore, it is inevitable that the dissolved oxygen will develop the native silicon oxide on the surface of a semiconductor wafer during cleansing in the use point 44.
It is a primary object of the present invention to provide a system for producing high-temperature ultrapure water for cleansing high density integrated circuit, which contains a minimal amount of dissolved oxygen, and to provide materials suitable for fabricating components and pipings which contact directly with the ultrapure water (referred to as materials hereinafter), more specifically, to provide a material for reducing the dissolved impurities in the ultrapure water to a negligible amount.